Creating a custom WHILE loop in NANT - nant

Question

I have tried my best to create a custom while loop but ended in vain.
Has anyone been successful in creating a custom while loop in NANT?

Answer 1

You can create a custom task :
<target name="sample">
<property name="foo.value" value="0"/>
<while property="foo.value" equals="0">
<do>
<echo message="${foo.value}"/>
<property name="foo.value" value="${int::parse(foo.value) + 1}"/>
</do>
</while>
</target>
<script language="C#" prefix="directory">
<code>
<![CDATA[
[TaskName("while")]
public class WhileTask : TaskContainer
{
private TaskContainer _doStuff;
private string _propertyName;
private string _equals;
private string _notEquals;
[BuildElement("do")]
public TaskContainer StuffToDo
{
get
{
return this._doStuff;
}
set
{
this._doStuff = value;
}
}
[TaskAttribute("property")]
public string PropertyName
{
get
{
return this._propertyName;
}
set
{
this._propertyName = value;
}
}
[TaskAttribute("equals")]
public string Equals
{
get
{
return this._equals;
}
set
{
this._equals = value;
}
}
[TaskAttribute("notequals")]
public string NotEquals
{
get
{
return this._notEquals;
}
set
{
this._notEquals = value;
}
}
protected override void ExecuteTask()
{
while (this.IsTrue())
{
this._doStuff.Execute();
}
}
private bool IsTrue()
{
if (!string.IsNullOrEmpty(this.Equals))
{
return this.Properties[this.PropertyName] == this.Equals;
}
return this.Properties[this.PropertyName] != this.NotEquals;
}
}
]]>
</code>
</script>

Answer 2

looking at the list of currently available tasks for NAnt, it looks like while is no longer supported (http://nant.sourceforge.net/release/latest/help/tasks/)
So I think the easiest and most efficient way how to do a custom while loop is recursion.
So for example, something like this:
<property name="count" value="120" />
<target name="wait">
<if test="${int::parse(count) > 0}" >
<property name="count" value="${int::parse(count) - 1}" />
<call target="wait"/>
</if>
</target>
Regards,
Marek

Answer 3

Here's another example of a simple yet effective version of the while loop implemented in NAnt.
<?xml version="1.0"?>
<project name="whiletask" xmlns="http://tempuri.org/nant-donotuse.xsd">
<script language="C#" prefix="loop">
<code>
<![CDATA[
/// <summary>
/// A while loop task. Will continuelly execute the task while the <c>test</c> is <c>empty</c>
/// or evalutes to <c>true</c>.
/// </summary>
[TaskName("while")]
public class WhileTask : TaskContainer
{
private string _test;
private TaskContainer _childTasks;
/// <summary>
/// The expression to test each iteration. If empty, then always evalutes to true (i.e. infinite loop.)
/// </summary>
[TaskAttribute("test", ExpandProperties = false)]
public string Test
{
get { return _test; }
set { _test = NAnt.Core.Util.StringUtils.ConvertEmptyToNull(value); }
}
/// <summary>
/// Superficial to ensure the XML schema is rendered correctly for this task. It will get executed
/// if tasks exist within it.
/// </summary>
[BuildElement("do")]
public TaskContainer ChildTasks
{
get { return _childTasks; }
set { _childTasks = value; }
}
/// <summary>
/// Executes the while loop while the <c>test</c> evalutes to true or <c>test</c> is empty.
/// </summary>
protected override void ExecuteTask()
{
while (this.Test == null
|| bool.Parse(Project.ExpandProperties(this.Test, this.Location)))
{
if (this._childTasks != null)
{
this._childTasks.Execute();
}
else
{
base.ExecuteTask();
}
}
}
}
]]>
</code>
</script>
<property name="i" value="0" />
<while test="${int::parse(i) <= 10}">
<echo message="${i}" />
<property name="i" value="${int::parse(i)+1}" />
</while>
</project>

Answer 4

There's quite a few ways you can do this. I wrote something similar to Cao which triggers off a property being true, so the condition can be as complex as you like and if it's made dynamic the value is evaluated each loop which is handy when you are calling functions e.g. to check a file exists. I also added simple break and continue controls. It can also be run as an infinite loop with no attributes which can be useful either when you want lots of conditions to exit (in which case use 'if' with break/continue or - in my case - I wanted to run a task until it exceptioned and then handle is with failonerror or a trycatch block.
Here's a bit of Nant script which shows two ways to countdown from 10:
<property name="greaterthanzero" value="${int::parse(count) > 0}" dynamic="true"/>
<property name="count" value="10" />
<while propertytrue="greaterthanzero" >
<echo>CountDown = ${count}</echo>
<property name="count" value="${int::parse(count) - 1}" />
</while>
<property name="count" value="10" />
<while>
<if test="${int::parse(count) > 0}" >
<echo>CountDown = ${count}</echo>
<property name="count" value="${int::parse(count) - 1}" />
<continue/>
</if>
<break/>
</while>
And here's a real world example I use to wait until a lockfile is deleted:
<property name="count" value="0" />
<property name="lockfileexists" value="${file::exists(lockfile)}" dynamic="true"/>
<while propertytrue="lockfileexists" >
<sleep seconds="1" />
<property name="count" value="${int::parse(count) + 1}" />
<if test="${count == '15'}" >
<echo>Timed out after 15 seconds</echo>
<break/>
</if>
</while>
Here's the task code:
<script language="C#" prefix="loops">
<code>
<![CDATA[
public class LoopBreakException : Exception {}
public class LoopContinueException : Exception {}
[TaskName("break")]
public class BreakTask : Task
{
protected override void ExecuteTask()
{
throw new LoopBreakException();
}
}
[TaskName("continue")]
public class ContinueTask : Task
{
protected override void ExecuteTask()
{
throw new LoopContinueException();
}
}
[TaskName("while")]
public class WhileTask : TaskContainer
{
[TaskAttribute("propertytrue")]
public string PropertyName { get; set; }
protected bool CheckCondition()
{
if (!string.IsNullOrEmpty(PropertyName))
{
try
{
return bool.Parse(Properties[PropertyName]);
}
catch (Exception ex)
{
throw new BuildException(string.Format("While Property '{0}' not found", PropertyName), Location);
}
}
//for infinite loops
return true;
}
protected override void ExecuteTask()
{
while (CheckCondition())
{
try
{
ExecuteChildTasks();
}
catch (LoopContinueException)
{
continue;
}
catch (LoopBreakException)
{
break;
}
}
}
}
]]>
</code>

Answer 5

Without additional information, there is a tutorial on creating a custom NAnt task here.
One nice thing about the article is the author suggests 2 means of debugging your custom task:
Copy the assembly (and pdb) file to the NAnt bin directory. Open your solution in Visual Studio that contains the source for your task. Place your breakpoints. Go to the project properties and open the Debugging page. Change the Debug Mode to Program and the Start Application to the path to the NAnt executable (e.g. C:\Program Files\NAnt\bin\NAnt.exe). Then set the working directory and/or command line arguments so that NAnt will pick up your build file. Click run and away you go.
Place System.Diagnostics.Debbugger.Break(); in your code before the line you want to break on. Re-compile the project and copy the assembly (and pdb) to the NAnt bin directory. When you run your NAnt script you should get a popup box asking you to choose a debugger.
There is another tutorial here.
Alternately, can you express your problem in terms of a foreach?

Answer 6

I have created the custom task by myself. But it seems there are some issues in using nested loops in NANT.
Basically I'm trying to use nested loop. A while loop inside a foreach or a foreach inside another foreach. But in both instances the loop executes the current target & the target from which the current target is called for every iteration instead of the body inside the second loop.
Regards
Sarathy

Answer 7

Here is one way of writing a WHILE loop in nant, with no custom tasks or script element, taking advantage of failonerror="false" on a foreach loop.
<property name="n" value="10000" /><!-- this would be inefficient if "n" is very large -->
<property name="i" value="0" />
<foreach item="String" in="${string::pad-right(' ', int::parse(n), ',')}" delim="," property="val" failonerror="false" >
<if test="${int::parse(i) > 3}"><!-- put our exit condition here -->
<fail message="condition met, exit loop early" />
</if>
<echo message="i: ${i}" />
<property name="i" value="${int::parse(i) + 1}" />
</foreach>
The output from executing the WHILE loop above is as follows. Note that, due to the failonerror="false" the fail call does not terminate the script:
[echo] i: 0
[echo] i: 1
[echo] i: 2
[echo] i: 3
[foreach] myscript.nant(24,18):
[foreach] condition met, exit loop early
BUILD SUCCEEDED - 1 non-fatal error(s), 0 warning(s)
I based the WHILE loop above on how I build a FOR loop, which is a slightly simplified version of the above code:
<property name="n" value="5" />
<property name="i" value="0" />
<foreach item="String" in="${string::pad-right(' ', int::parse(n), ',')}" delim="," property="val" >
<echo message="i: ${i}" />
<property name="i" value="${int::parse(i) + 1}" /> <!-- increment "i" -->
</foreach>
The output from the FOR loop looks like this:
[echo] i: 0
[echo] i: 1
[echo] i: 2
[echo] i: 3
[echo] i: 4
BUILD SUCCEEDED